In the field of non-linear optics, the relationship between the polarization induced in a molecule (p) and the electric field components of incident electromagnetic waves (E) is approximately given by EQU p =.alpha..multidot.E +.beta..multidot..multidot.EE +.gamma..multidot..multidot..multidot.EEE +...
Since p and E are vector quantities, .alpha., .beta., .gamma., etc., are tensors.
A similar expression can be written for the polarization induced in an ensemble of molecules in the liquid, solid, or gas phase. In this case, with the electric field dipole approximations, the polarization P is written as EQU P =.chi..sup.(1) .multidot.E +.chi..sup.(2) .multidot..multidot.EE +.chi..sup.(3) .multidot..multidot..multidot.EEE +...
The coefficients .chi..sup.(1), etc., are tensors with similar meanings in relation to the molecular quantities, except that they describe the polarization induced in the ensemble.
There are several non-linear effects occurring through .chi..sup.(3). These include third harmonic generation (THG), optical bistability resulting from light-intensity-induced changes in the refractive index of the medium in a resonant cavity, and optical phase conjugation, which results from a degenerate four-wave mixing process by which two beams interfere to form a phase grating, and the complex conjugate of the phase front of the incoming beam is created as an outgoing beam. These effects are implemented as optical switching devices, optical modulators, and optical computing elements, for example.
Highly conjugated organic polymers typically have large non-resonant electronic susceptibilities (.chi..sup.(3)), which give the molecules unusual optical properties. It is desired to enhance these properties.
While some enhancement has been achieved by decreasing the bandgap of the material and increasing the bandwidths, such strategies do not lead to the significant enhancement desired.